Cyanine-like dyes with large bond-length alternation

Article abstract of DOI:10.1002/chem.201300609

Herein, the synthesis and properties of alkyne-bridged carbocations, which are analogous in structure to cyanine dyes, are reported. An alkene-bridged dye, linked at the third position of the indole, was also synthesized as a reference compound. These new carbocations are stable under ambient conditions, allowing characterization by UV/Vis and NMR (¹H and ¹³C) spectroscopies. These techniques revealed a large degree of delocalization of the positive charge, similar to a previously reported porphyrin carbocation. The linear and nonlinear optical properties are compared with cyanine dyes and triarylmethyl cations, to investigate the effects of the bond-length alternation and the overall molecular geometry. The value of Re(γ), the real part of the third-order microscopic polarizability, of -1.3×10^-33 esu for the alkyne-linked cation is comparable to that of a cyanine dye of similar length. Nondegenerate two-photon absorption spectra showed that the alkene-bridged dye exhibited characteristics of cyanines, whereas the alkyne-bridged dye is reminiscent of octupolar chromophores, such as the triarylmethyl carbocation brilliant green. Such attributes were confirmed and rationalized by quantum chemical calculations. Copyright

Full text of DOI:10.1002/chem.201300609

FULL PAPER
Nonlinear Optics
K. J. Thorley, J. M. Hales, H. Kim,
S. Ohira, J.-L. Brꢀdas,* J. W. Perry,*
H. L. Anderson* ............. &&&& — &&&&
Cyanine-Like Dyes with Large Bond-
Length Alternation
Like cyanines, yet different: New cati-
onic dyes, which have alkyne bridges
between the indole end groups, have
been synthesized. Despite the large
bond-length alternation (BLA), these
carbocations display cyanine-like non-
linear optical behavior, such as large
negative third-order hyperpolarizabili-
ties, although exhibiting two-photon
absorption behavior similar to that of
triarylmethyl carbocations, such as bril-
liant green (see figure).
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DOI: 10.1002/chem.201300609
Cyanine-Like Dyes with Large Bond-Length Alternation
Karl J. Thorley,[a] Joel M. Hales,[b] Hyeongeu Kim,[b] Shino Ohira,[b] Jean-Luc Brꢀdas,*[b]
Joseph W. Perry,*[b] and Harry L. Anderson*[a]
Abstract: Herein, the synthesis and
properties of alkyne-bridged carbocat-
viously reported porphyrin carbo
ACHTUNGTRENNiUNG on. The linear and nonlinear optical
G
alkyne-linked cation is comparable to
that of a cyanine dye of similar length.
Nondegenerate two-photon absorption
spectra showed that the alkene-bridged
dye exhibited characteristics of cya-
nines, whereas the alkyne-bridged dye
is reminiscent of octupolar chromo-
phores, such as the triarylmethyl carbo-
cation brilliant green. Such attributes
were confirmed and rationalized by
quantum chemical calculations.
A
properties are compared with cyanine
dyes and triarylmethyl cations, to in-
vestigate the effects of the bond-length
alternation and the overall molecular
geometry. The value of Re(g), the real
part of the third-order microscopic po-
larizability, of ꢀ1.3ꢂ10ꢀ33 esu for the
to cyanine dyes, are reported. An
alkene-bridged dye, linked at the third
position of the indole, was also synthe-
sized as a reference compound. These
new carbocations are stable under am-
bient conditions, allowing characteriza-
tion by UV/Vis and NMR (1H and 13C)
spectroscopies. These techniques re-
vealed a large degree of delocalization
of the positive charge, similar to a pre-
Keywords: alkynes · carbocations ·
cyanines · nonlinear optics
Introduction
Cyanine dyes have been studied extensively due to their ex-
ceptional linear and nonlinear optical (NLO) properties,
leading to applications in biological imaging,[1] optical-data
storage,[2] and all-optical switching.[3] For example, extended
cyanine dyes tend to have a large negative real part of the
third-order polarizability, Re(g), in the telecommunication
wavelength range (l=1.3–1.6 mm), making them relevant
for all-optical switching applications, although having low
values of the imaginary third-order polarizability, Im(g),
thus minimizing optical losses by two-photon absorption
(2PA).[3] The strong electronic delocalization and intense
low energy absorption bands of cyanine dyes are associated
with a lack (or very limited extent) of bond-length alterna-
tion.[4] Recently, in a computational study, we identified
model alkyne carbocations with large bond-length alterna-
tion, which exhibited cyanine-like properties (Figure 1).[5]
Figure 1. Cyanine and alkyne–cyanine structures.
Subsequently, Jacquemin compared the polarizabilities of
the “alkynes–cyanines” to those of conventional cyanines, at
the DFT level.[6] However, experimental characterization of
the NLO properties of the “alkyne–cyanines” had not been
carried out to date, due to the lack of suitably stable alkyne
carbocationic systems.
Alkyne-bridged carbocations similar to the structure
shown in Figure 1 have been reported by Nakatsuji et al.[7]
Extension of one arm of a triarylmethyl carbocation with an
ethynyl group resulted in a redshift of the absorption spec-
trum. Extension of more than one arm caused a large de-
crease in stability and gave a pyrylium cation decomposition
product. Komatsu et al. successfully measured linear absorp-
tion and NMR spectra of similar dyes,[8] but such experi-
ments were only possible at low temperature or over time
periods of seconds. Arisandy et al. reported the synthesis of
ferrocenyl cations with two cyclopentadiene units linked by
either an alkene or alkyne bridge;[9] although the alkene-
linked cations were very stable, the alkyne-linked cations
rapidly decomposed to give unidentifiable products. A
ruthenium-stabilized ethynyl cation, which was stable under
anaerobic conditions, has also been reported.[10]
[a] Dr. K. J. Thorley, Prof. H. L. Anderson
Department of Chemistry, University of Oxford
Chemistry Research Laboratory
Mansfield Road, Oxford, OX1 3TA (UK)
Fax: (+44)1865-285-002
E-mail: harry.anderson@chem.ox.ac.uk
[b] Dr. J. M. Hales, H. Kim, Dr. S. Ohira, Prof. J.-L. Brꢁdas,
Prof. J. W. Perry
School of Chemistry and Biochemistry
Georgia Institute of Technology
Atlanta, GA 30332-0400 (USA)
E-mail: jean-luc.bredas@chemistry.gatech.edu
joe.perry@gatech.edu
Herein, we present the synthesis of alkyne-bridged cya-
nine analogues and explore the effect of the inherent bond-
length alternation and chemical structure on the molecular
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300609.
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Cyanine-Like Dyes with Large Bond-Length Alternation
FULL PAPER
properties. The resulting dyes are stable for long periods of
time under ambient conditions, which allowed us to investi-
gate their properties by using a wide range of techniques.
13C NMR analysis of isotopically enriched cations provided
insight into the degree of charge delocalization. The linear
and nonlinear optical properties were compared with an
alkene-linked carbocation, as well as commercially available
cyanines and triarylmethyl carbocations, by using femtosec-
ond-pulsed Z-scan and nondegenerate 2PA techniques.
Figure 2. Normalized absorption spectra of alkynyl cations 7a+ (solid
line) and 7b+ (dashed line) recorded in CHCl3/2% TFA at 298 K.
Results and Discussion
Our first approach to synthesizing alkyne–cyanines followed
the route shown in Scheme 1 (compounds 1a–7a+; R=H).
Regioselective 3-iodination of indole 1a was carried out as
To facilitate photophysical studies, it was necessary to syn-
thesize a more stable version of the alkyne–cyanine. This
was achieved by incorporation of a 2-phenyl substituent on
the indole (Scheme 1, compounds 1b–7b+; R=Ph). The ad-
ditional phenyl groups stabilize the cationic species, proba-
bly by hindering nucleophilic attack. The silyl-protected
2-phenyl indole could not be separated from iodoindole 3b
and from Sonogashira coupling by-products due to the high
mobility of these compounds on silica; therefore, an alco-
hol-protecting group was used (4b). The disadvantage of the
new protecting group was the lower yield in the deprotec-
tion step. The alkyne–cyanine was generated in solution by
addition of trifluoroacetic acid (TFA; 2%) to a solution of
6b in CHCl3. The near-IR absorption band of 7b+ is red-
shifted by approximately 50 nm from that of 7a+. By moni-
toring the absorption over time, it was seen that the 2-Ph
alkyne–cyanine is substantially more stable than the 2-H
1
alkyne–cyanine.[12] The cation was still observed by H NMR
and absorption spectroscopy after one week in solution in
CHCl3/2% TFA at about 50% of the original concentration.
As well as forming the stable carbocation 7b+ in solution
for absorption and NMR experiments, the tetrafluoroborate
salt was isolated as a solid in a high yield (87%). Addition
of tetrafluoroboric acid diethyletherate complex to a solu-
tion of alcohol 6b in Et2O resulted in the precipitation of
Scheme 1. Synthesis of alkyne–cyanines 7a+ and 7b+. a) I2, KOH, DMF;
2a 55%; b) MeI, NaH, DMF; 3a 88%, 3b 98%; c) for 4a: HCCSiMe3,
[Pd
butyn-2-ol, [Pd
CH2Cl2, 99%; for 5b: NaOH, PhMe, 1008C, 50%; e) LiNAHCTUNGTRENNUNG
(dba)3], PPh3, CuI, PhMe, NEt3, 408C, 40%; for 4b: 2-methyl-3-
(PPh3)2Cl2], CuI, DMF, NEt3, 61%; d) for 5a: Bu4NF,
(SiMe3)2,
ACHTUNGTRENNUNG
the carbocation salt. The absorption spectrum of 7b+BF4 is
ꢀ
PhCO2Me, THF; 6a 78%, 6b 95%; f) CF3CO2H, CHCl3, or HBF4·Et2O,
Et2O, 87 % (7b+).
ꢀ
almost identical to that 7b+CF3CO2 formed in situ, but the
1H NMR spectrum of the tetrafluoroborate salt is signifi-
cantly broader. Upon addition of [D]TFA to a solution of
was reported by Bocchi and Palla.[11] N-Methylation proved
essential to the stability of the iodinated indole. Sonogashira
coupling under standard conditions gave the alkynyl indole
4a. Alcohol 6a was synthesized directly from the deprotect-
ed ethynyl indole and methyl benzoate. The presence of a
base (NEt3) in the chromatography eluent was necessary to
avoid decomposition of these alkynes on silica.
ꢀ
7b+BF4 in CDCl3, the spectrum became identical to that of
ꢀ
7b+CF3CO2 .
1
ꢀ
The H NMR spectrum of 7b+CF3CO2 showed the pres-
ence of only one species. The spectrum was fully assigned
by using a range of 2D NMR techniques. In general, the
13C NMR spectrum of a carbocation provided information
on the delocalization of the positive charge, because cationic
Alkyne–cyanine 7a+ was formed by addition of trifluoro-
acetic acid (2%; v/v) to a solution of alcohol 6a in chloro-
form. Initially, the absorption showed an intense cyanine-
like band at l=750 nm (Figure 2). This absorption band
became less intense over time, and there was a concomitant
shift in the absorption band to around l=550 nm. By moni-
toring the intensity of the l=750 nm absorption band over
time, the rate of decomposition was found to depend on the
acid concentration.[12]
centers are strongly deshielded, for example, the central
+
carbon of C
N
nance-stabilized carbocations do not exhibit such extreme
chemical shifts.[13] Unfortunately, 2D NMR correlation ex-
periments did not allow us to assign all the 13C resonances
of 7b+ due to the lack of protons in proximity to the central
carbon atoms. A 13C-enriched analogue of 6b was synthe-
sized by using labeled methyl benzoate and indole 5b. The
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J.-L. Brꢁdas, J. W. Perry, H. L. Anderson et al.
13C-labelled cation 7b+ was formed by addition of acid. By
comparison of the spectra of naturally abundant and 13C-en-
riched alkyne–cyanines, the central labeled carbon was as-
signed, and also the adjacent carbons through their coupling
to the 13C label. The change in chemical shift between the
cation and the precursor alcohol (DdC) is a useful guide to
charge delocalization. Comparison of these shifts with previ-
ously reported compounds with similar bridge structures
(Table 1) revealed that the extent of charge delocalization in
Table 1. 13C NMR chemical shifts (dC) and changes in chemical shift
(DdC) for alkynyl carbocations compared with the corresponding alcohol
precursors, at carbon centers 1–3.
Cation
dC1 (DdC1
)
dC2 (DdC2
)
dC3 (DdC3
)
Reference
Scheme 2. Synthesis of reference cation 14+: a) POCl3, DMF, 408C, 89%;
b) MeI, NaH, DMF, 99%; c) BrCH2PPh3Br, KOtBu, THF, 81%;
d) nBuLi, PhCO2Me, THF, ꢀ788C, 5%; e) CF3CO2H, CHCl3.
7b+
8+
139.0 (72.4)
162.2 (96.4)
133.9 (66.3)
117.2 (25.2)
106.5 (17.4)
118.9 (22.8)
134.5 (54.6)
145.5 (60.6)
136.1 (49.5)
this work
[8]
9+
[14]
1
stereochemistry of 13 was provided by its simple H NMR
spectrum and J=16 Hz coupling constant.
The linear absorption spectrum of the alkene-linked
cation 14+ showed an 80 nm blueshift from the alkyne-
linked cation 7b+ (Table 2). The absorption maxima of 7b+
ꢀ
Table 2. Linear and nonlinear optical properties of cations 7b+CF3CO2
ꢀ
and 14+CF3CO2 (in CHCl3/2% TFA), and cyanines 16+Iꢀ and 17+Iꢀ (in
CH2Cl2). The nonlinear optical properties were measured by the Z-scan
method at l=1300 nm.
Dye
lmax [nm]
e [mꢀ1 cmꢀ1
]
Re(g)[a] [esu][b]
|Re(g)/Im(g)|[a]
7b+
14+
16+
17+
785
704
652
756
76000
69000
220000
320000
ꢀ1.0ꢂ10ꢀ33
ꢀ2.3ꢂ10ꢀ33
NA[c]
1.5
1.0
NA[c]
2.1
ꢀ8.4ꢂ10ꢀ33
indole-terminated alkyne–cyanine 7b+ is much greater than
in a simple dialkynyl cation 8+ and similar to the porphyrin
dimer 9+, despite the much smaller size of the p system in
7b+.[8,14] The observation that the 13C chemical shift is great-
er on C1 than C2 or C3 indicates that C1 has the greatest
positive charge in 7b+ and 8+; however, localization on C1
is substantially less in 7b+ than in 8+.
The alkyne–cyanines 7a and b are terminated with
3-indole electron donors. Most previously reported indole-
based cyanines are connected at the second position of the
indole, although a few examples of trimethine–cyanines sub-
stituted at the third position have been reported.[15] To
ensure proper comparisons, we set out to synthesize a three-
substituted pentamethine reference compound with a cen-
tral phenyl group (Scheme 2). Vilsmeier formylation[16] of
2-phenyl indole and N-methylation gave aldehyde 11, which
was transformed to vinyl bromide 12 by a Wittig coupling,[17]
giving a mix of cis- and trans-isomers in a 1:2 ratio. Halo-
gen–lithium exchange and subsequent trapping with methyl
benzoate gave alcohol 13 in poor yields (it is essential to
keep the reaction mixture cold during this step; at tempera-
tures above ꢀ788C, 12 undergoes elimination to give 5b,
leading to formation of the acetylene–cyanine precursor 6b
as an inseparable by-product). Evidence for the trans,trans-
[a] Errors for Re(g) were estimated to be ꢁ10%; errors for |Re(g)/
Im(g)| were estimated to be ꢁ14%. [b] 1 esu=1 cm2 statVoltꢀ2. [c] No
closed-aperture signal was measured, thus, Re(g) was found to be negligi-
ble.
and 14+ (l=785 and 704 nm) are similar to those of the
known second-position-linked indocyanine dyes (l=652 for
Cy5 16+ and 756 nm for Cy7 17+; structures are shown in
Figure 3). However, the molar extinction coefficients of the
new cations are much lower than the two cyanines (Table 2),
as was reported for similar third-position-substituted indoc-
yanine dyes.[15c]
The NLO properties of the chromophores were also in-
vestigated. The real part of g, Re(g), and its ratio to the
imaginary part of g, jRe(g)/Im(g)j were determined at l=
1300 nm by using the femtosecond-pulsed Z-scan technique
(results are summarized in Table 2).[12] The values of Re(g)
for nearly all the compounds exhibited characteristics that
are typical of cyanines: they are large in magnitude and neg-
ative in sign. Although this was expected for the second-
and third-position-substituted indocyanines, observation of
similar characteristics for 7b+ is entirely consistent with the
theoretical results[5] that showed model alkyne-carbocations
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Cyanine-Like Dyes with Large Bond-Length Alternation
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Figure 3. Structures of cyanines 16+ and 17+, and triarylmethyl cations
crystal violet 18+ and brilliant green 19+ used as reference compounds
for 2PA.
exhibited cyanine-like properties. On the contrary, it should
be noted that 16+ exhibited an unexpectedly low (negligi-
ble) value of Re(g) at l=1300 nm. To rationalize this result,
it is important to note that the value of Re(g) can undergo
significant dispersion near a 2PA resonance. On the high-
energy side of the resonance, a strong negative dip in Re(g)
occurred, whereas on the low-energy side, there was a
strong positive peak. The 2PA spectrum of 16+ showed that
excitation at l=1300 nm, which is on the low energy side of
the 2PA resonance (see below). This leads to reduction of
the expected large, negative, Re(g) by the competing posi-
tive dispersion peak, leading to the negligible value of
Re(g) observed by Z-scan measurements. The 2PA spectra
of the remaining compounds indicated that 1300 nm resides
on the high-energy side of the 2PA resonance. Consequently,
one would expect enhancements of the intrinsic, negative
Re(g) values. This is consistent with results from a simplified
two-state model for zero-frequency Re(g),[3] in which one
would expect that the carbocations (7b+ and 14+), given
their weaker e values, should possess Re(g) values with sig-
nificantly smaller magnitudes than cyanines 16+ and 17+.
Instead, comparable values of Re(g) were observed for the
carbocations due to this two-photon-dispersion enhance-
ment.
Nondegenerate two-photon absorption (ND-2PA) spectra
of cations 7b+, 14+, 16+, 17+, 18+, and 19+ were recorded by
using an ultrafast pump-probe spectroscopy set-up. The ND-
2PA spectra are shown in Figures 4, 5, and 6, overlaid with
the normalized one-photon absorption (1PA) spectra. This
technique probes 2PA by combining a tunable pump pulse
(120 fs, l=700–2200 nm) with a broadband white-light con-
tinuum probe pulse (l=420–850 or 850–1650 nm, latter des-
ignated as NIR).[12] The sum of the photon energies from
the two pulses allowed access to relevant two-photon transi-
tions. The broadband nature of the probe pulse, coupled
with the use of different pump wavelengths (shown in the
legends) allowed coverage of the entire 2PA spectrum. The
abscissa in each figure reflects the combined photon ener-
Figure 4. ND-2PA spectra (circles) of cyanines 16+Iꢀ (top) and 17+Iꢀ
(bottom) recorded in CH2Cl2. The legend gives the pump wavelength em-
ployed. The solid lines show the 1PA spectra. The crosses show the de-
generate 2PA cross-sections determined by Z-scan.
gies of the two beams involved in the 2PA process, to give
an overall transition wavelength. The left ordinate denotes
the
ND-2PA
cross-section,
dND
(1 GM=1ꢂ
10ꢀ50 cm4 sphotonꢀ1). The sharp peaks in these ND-2PA
spectra are due to inverse Raman scattering present in the
solvent and chromophore; however, these signatures are
spectrally narrow and therefore distinguishable from the
broadband ND-2PA response. The ND-2PA technique was
chosen, because it provides rapid and broadband characteri-
zation and is amenable for studying nonfluorescent samples.
However, for comparison purposes, degenerate 2PA (D-
2PA) cross-sections (dD) at selected wavelengths were deter-
mined from open-aperture Z-scans and two-photon-induced
fluorescence (2PF).[12] These values (right ordinates), also
shown in Figures 4–6, indicate that there is good agreement
between the three methods.
The ND-2PA spectra of 16+ and 17+ illustrate a number
of characteristics typical for polymethine–cyanines
(Figure 4). First, the lower energy 2PA peak is spectrally co-
incident with a vibronic shoulder of the lowest energy one-
photon state at E10. The vibronic coupling of the lowest-
lying 1PA-active state appears to be fairly strong, as has
been observed both in other cyanines (experimentally[3,18]
and theoretically[18b]) and in squaraines (experimentally[19]
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J.-L. Brꢁdas, J. W. Perry, H. L. Anderson et al.
ꢀ
Figure 6. ND-2PA spectra (circles) of carbocations 7b+CF3CO2 (top)
Figure 5. ND-2PA spectra (circles) of triarylmethyl carbocations crystal
ꢀ
violet 18+Clꢀ (top) and brilliant green 19+HSO4 (bottom) recorded in
ꢀ
and 14+CF3CO2 (bottom) recorded in CHCl3/2% TFA. The legend
MeCN. The legend gives the pump wavelength employed. “NIR” refers
to use of the near-infrared probe region. The solid lines show the 1PA
spectra. The crosses show the degenerate 2PA cross-sections determined
by two-photon-excited fluorescence.
gives the pump wavelength employed. The solid lines show the 1PA spec-
tra. The crosses show the degenerate 2PA cross-sections determined by
Z-scan.
knowledge, this represents the first experimental measure-
ment of broadband 2PA spectra for these known octupolar
molecules, and it reveals a number of interesting spectral
characteristics. In particular, the longest wavelength 2PA
band is coincident with the lowest-energy one-photon state
at E10 (e.g., for 19+ this occurs at lꢂ630 nm), consistent
with the fact that both one- and two-photon transitions into
this 1E’ state are allowed in these compounds.[21] Additional-
ly, the short wavelength 2PA peaks are associated with the
2A’ excited states in these compounds and occur at slightly
lower energy than for cyanines (ꢂ1.5ꢂE10). It is also inter-
esting to note that in the 2PF spectra vibronically allowed
2PA is observed into E10 (e.g., for 19+ this occurs at
ꢂ575 nm) as in cyanines, albeit with a reduced response
(unfortunately, in the case of 18+, the vibronic 2PA is not
clearly discernible in the ND-2PA spectrum). This two-
photon response has been predicted theoretically for crystal
violet 18+ and the experimentally determined dispersion of
the first hyperpolarizability has also supported such vibronic
activity.[21] The ND-2PA spectrum of the alkyne carbocation
7b+ exhibits all the spectral characteristics described above
for the triarylmethyl carbocations, particularly for brilliant
green (19+), the compound with reduced symmetry
and theoretically[20]). Second, higher-energy 2PA peaks were
observed at approximately 1.7ꢂE10, again typical for previ-
ously investigated cyanines.[3,18a] The ND-2PA spectrum of
the alkene-bridge model carbocation 14+ was found to be
similar to that of these cyanines (Figure 6) with pronounced
vibronically allowed 2PA lying at approximately 1200 cmꢀ1
above the lowest lying 1PA-active state. Some difficulties
encountered with absorption of shorter wavelength pump
beams prohibited access to the higher-energy 2PA bands for
this compound.
The carbocation 7b+ may be considered as an extended
version of triarylmethyl carbocations, such as brilliant green
19+ and crystal violet 18+ (Figure 3) with the latter possess-
ing a higher degree of symmetry. The 2PA spectra of 18+
and 19+ have been previously calculated, and the D-2PA
cross-sections were measured by 2PF at selected wave-
lengths.[21] The reported cross-sections are shown in Figure 5
as crosses, together with the broadband 2PF data for Bril-
liant Green 19+ from the current study (2PF data could not
be obtained for 18+ due to the very low quantum yield.)
The ND-2PA spectra of 18+ and 19+ agree well with the
2PF data from this work and Ref. [21]. To the best of our
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Cyanine-Like Dyes with Large Bond-Length Alternation
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(Figure 6). It is also interesting to note that the vibronically
allowed 2PA for 7b+ lies at approximately 1700 cmꢀ1 above
the lowest lying 1PA active state, as was expected for an
alkyne-bridged system, in contrast to the alkene-bridged
polymethines or aryl-bridged octupolar molecules.
Quantum chemical calculations were performed on the
alkene/alkyne carbocations to help rationalize and corrobo-
rate the observations reported above.[12] In quadrupolar and
octupolar compounds, which consist of a combination of
electron donor (D) and acceptor (A) segments, two-photon
cross-sections are enhanced with electron-density shift be-
tween the D-p-A moieties. Beljonne et al. underlined that
the increase in the cross-sections of octupolar compounds
are accounted for by donor-to-acceptor charge transfer in
the three arms.[22] Figure 7 illustrates the calculated charge
arm of 14+, because it is twisted out of plane by about 558;
this implies that the phenyl substituent makes a small contri-
bution to the 2PA response of 14+. The S3 state of 18+ is
very weakly active (likely, vibronically allowed) in 1PA, but
is strongly 2PA active, whereas the charge transfer (S2)
states of 7b+ and 19+ are both 1PA and 2PA active, as was
seen in the UV/Vis and ND-2PA spectra, due to greater
lowering of the symmetry in 7b+ and 19+ relative to 18+.
Finally, we note that it was shown earlier in the case of
squaraine compounds that the lowest-energy 2PA activity
was not related to the presence of a 2PA-active electronic
state; rather, it could be explained through coupling of the
1PA-active S1 state of these compounds with b(u) vibration-
al modes.[20,23] For the model cyanine and alkyne–cyanine
structures reported herein, the ND-2PA spectra exhibited
significant low-lying 2PA activity some 0.2–0.25 eV above
the lowest 1PA-active state (S1), which is at a much lower
energy than the one calculated for the lowest-energy strong-
ly active 2PA state. These results strongly suggest that the
lowest-energy 2PA activity in the present model compounds
has also a vibronic origin.
Conclusion
The alkyne-linked cyanine analogue 7b+ was synthesized
and found to be sufficiently stable for spectroscopic analysis
under ambient conditions. Despite possessing a large bond-
length alternation, the compound exhibited characteristics
indicative of traditional cyanines. Absorption spectroscopy
showed a strong near-IR band at l=785 nm and a weaker
visible band at 550 nm. The extinction coefficient of the
near-IR band is smaller than for a typical cyanine (such as
16+ or 17+), but comparable to previously reported indoli-
um dyes substituted on the third position. 13C NMR spectro-
scopy of an isotopically enriched cation was used to investi-
gate the extent of delocalization across the alkynyl bridge.
The change in chemical shift upon cation formation indicat-
ed a large degree of charge delocalization, comparable to
that in the previously reported porphyrin cation 9+.
Figure 7. Illustration of the charge transfer occurring upon excitation
from the S0 ground state to the strongly 2PA-active state of 7b+ (S2), 14+
(S2), 18+ (S3), and 19+ (S2) at the INDO/MRDCI level. Open circles
denote the loss of charge and filled circles the gain of charge upon photo-
excitation.
transfer that occurs upon going from the ground state to the
excited state with high 2PA response (S2 or S3 state in
Table 3) for cations 7b+, 14+, 18+, and 19+. As in cations
18+ and 19+, the planar arrangement of 7b+ allows charge
transfer between each of the three aromatic arms. However,
there is no charge transfer taking place along the phenyl
A vinylene-linked 3-indole cyanine cation 14+ was synthe-
sized to compare its optical properties with alkyne-cyanine
7b+. The values of the real part of the third-order polariza-
bility, Re(g), for 14+ and 7b+ are comparable to those of
traditional cyanines, particularly when accounting for two-
photon dispersion enhancement effects. ND-2PA measure-
ments confirm that the alkene carbocation 14+ has cyanine-
like characteristics, such as vibronically allowed 2PA into
the lowest-lying 1PA-active state. On the other hand, the
alkyne carbocation 7b+ exhibits characteristics indicative of
triarylmethyl carbocation dyes, such as a low-energy 2PA
band coincident with a 1PA-allowed transition, similar to
the 2PA spectrum of brilliant green (19+).
Table 3. Electronic and optical properties of 7b+, 14+, 18+, and 19+ cal-
culated at the INDO/MRDCI//SOS level.
Sn
E [eV]
f
m0n [D]
m1n [D]
d [GM]
S1
S2
S1
S2
S1
S2
S3
S1
S2
1.65
2.55
1.43
2.72
1.90
1.98
3.60
1.84
2.80
0.99
0.15
1.21
0.04
0.48
0.49
0.01
0.74
0.31
12.6
3.9
14.9
2.0
8.2
8.0
0.0
10.3
5.4
–
8.4
–
8.2
–
–
32
7b+
14+
2.5ꢂ103
11
1.1ꢂ105
78
18+
19+
97
(m13/m23) 7.5/7.9
1.0ꢂ105
27
–
9.0
1.7ꢂ103
Chem. Eur. J. 2013, 00, 0 – 0
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J.-L. Brꢁdas, J. W. Perry, H. L. Anderson et al.
tion by silica chromatography (40–608C petroleum ether/NEt3 1:0.02)
and removal of solvents gave the product (0.164 g, 95%) as a yellow-
brown solid. M.p. 70–768C; 1H NMR (500 MHz, CDCl3): d=7.81–7.78
(4H, m), 7.61 (4H, d, J=8.3 Hz), 7.47–7.38 (8H, m), 7.34–7.30 (5H, m),
7.25 (2H, dd, J=6.9, 7.8 Hz), 3.76 ppm (6H, s); 13C NMR (125 MHz,
CDCl3) d 144.5, 142.7, 137.1, 130.6, 130.2, 129.0, 128.5, 128.4, 128.1, 128.1,
126.2, 122.9, 120.9, 120.2, 109.8, 95.8, 91.9, 79.9, 66.7, 31.6 ppm; HRMS
(ESI): m/z calcd for C41H30N2+: 549.2325 ([M]+); found: 549.2328.
Synthesis of 2-phenyl acetylene–cyanine trifluoroacetate (7b+): The ace-
tylene–cyanine was formed in situ for NMR and absorption experiments
by addition of trifluoroacetic acid (2% by volume with respect to sol-
vent) to a solution of the alcohol precursor 6b in CHCl3. 1H NMR
(500 MHz, CDCl3/2% [D]TFA): d=7.84 (2H, d, J=6.9 Hz), 7.81 (2H,
dd, J=7.1, 6.9 Hz), 7.72–7.77 (8H, m), 7.55–7.64 (9H, m), 7.34 (2H, dd,
J=7.9, 7.7 Hz), 3.97 ppm (6H, s); 13C NMR (125 MHz, CDCl3/2%
[D]TFA): d=156.9, 139.1, 139.0, 136.0, 135.3, 134.5, 131.6, 130.8, 130.7,
129.4, 129.4, 128.0, 127.8, 127.0, 125.6, 121.2, 117.8, 117.2, 115.6, 113.3,
112.3, 111.0, 101.4, 99.9, 33.0 ppm; UV/Vis (CHCl3/2% TFA): lmax
(loge)=536 (4.20), 785 nm (4.88).
Experimental Section
General: Reactions were carried out under nitrogen or argon atmo-
ACHTUNGTRENNUNGspheres, unless otherwise stated. Dry toluene and THF were used after
passing through a column of activated alumina. Triethylamine was distil-
led from CaH2 prior to use. All other reagents were used as supplied.
Column chromatography was carried out on silica gel 60 under a positive
pressure of nitrogen. Ratios are reported by volume when mixtures of
solvents were used. NMR spectra were recorded at 300 K on a Bruker
DPX400 (400 MHz) or Bruker AVANCE AVC500 (500 MHz) spectrom-
eter. Chemical shifts are quoted as parts per million (ppm) relative to tet-
ramethylsilane. UV/Vis spectra were recorded in solution on a Perkin–
Elmer lambda 20 UV/Vis spectrometer. Mass spectra were measured by
electrospray ionization time-of-flight (ESI-TOF) by using a Micromass
LCT premier spectrometer. Only molecular ions and major peaks are re-
ported.
Synthesis of 3-iodo-1-methyl-2-phenylindole (3b): A one-pot method was
adopted by combining published procedures.[10,24] 2-Phenylindole (1b;
1.00 g, 5.18 mmol) was dissolved in DMF (40 mL). Ground potassium hy-
droxide (0.726 g, 12.9 mmol) was added and the solution turned green. A
solution of iodine (1.33 g, 5.23 mmol) in DMF (20 mL) was added drop-
wise, until the brown color persisted. Sodium hydride (60% dispersion,
0.248 g, 6.22 mmol) was added and the mixture stirred for 10 min. Iodo-
methane (0.946 mL, 6.22 mmol) was added, and the reaction mixture was
stirred at 208C for 1 h, then quenched with H2O (20 mL), extracted with
EtOAc (40 mL), and washed with H2O (3ꢂ40 mL). The solution was
dried over magnesium sulfate. Evaporation of the solvents gave the prod-
uct (1.70 g, 98%) as a yellow oil. 1H NMR (400 MHz, [D6]DMSO): d=
7.55–7.44 (6H, m), 7.32 (1H, d, J=7.7 Hz), 7.26 (1H, t, J=7.0 Hz), 7.17
(1H, dd, J=7.2, 7.6 Hz), 3.60 ppm (3H, s).
Synthesis of 2-phenyl-1-methylindole-3-carbaldehyde (11): Formyl indole
was methylated following
(60% dispersion in oil, 0.022 g, 0.54 mmol) was added to formyl indole
10 (0.100 g, 0.45 mmol) in DMF (2 mL) and stirred for 10 min. Iodometh-
AHCTUNGERTGaNNUN ne (0.082 mL, 0.54 mmol) was added and the reaction mixture and stir-
a
published procedure.[24] Sodium hydride
ACHTUNGTRENNUNG
red for 1 h. H2O (2 mL) was added to quench the reaction, and the prod-
uct was extracted with EtOAc (10 mL), washed with H2O (2ꢂ10 mL),
and dried over magnesium sulfate. Evaporation of solvents gave the
product as
a
yellow powder (0.104 g, 99%). 1H NMR (400 MHz,
[D6]DMSO): d=9.96 (1H, s), 8.23 (1H, d, J=7.7 Hz), 7.60–7.69 (7H, m),
7.39 (1H, dd, J=7.0, 8.2 Hz), 7.33 (1H, dd, J=7.8, 7.2 Hz), 3.70 ppm
(3H, s); HRMS (ESI): m/z calcd for C16H13NNaO: 258.0889 ([M]+);
found: 258.0890.
Synthesis of 2-methyl-4-(1-methyl-2-phenyl-indol-3-yl)but-3-yn-2-ol (4b):
Sonogashira coupling was achieved by following a published proce-
dure.[25] 2-Ph-N-Me iodoindole 3b (1.78 g, 5.38 mmol), bis(triphenylphos-
phine)palladium(II) dichloride (0.189 g, 0.270 mmol) and copper(I)
iodide (0.102 g, 0.538 mmol) were dried under vacuum. Triethylamine
(53 mL) and DMF (35 mL) were added, and the solution was freeze/thaw
degassed. 2-Methyl-3-butyn-2-ol (1.04 mL, 10.7 mmol) was added, and
the mixture was stirred for 18 h at 208C. The solvents were removed, and
the product purified was by silica chromatography (40–608C petroleum
ether/CH2Cl2/NEt3 20:4:1) to give product as a yellow oil (0.949 g, 61%).
1H NMR (400 MHz, CDCl3): d=7.74 (1H, d, J=7.7 Hz), 7.62 (2H, d, J=
7.0 Hz), 7.52 (2H, dd, J=7.0, 7.9 Hz), 7.45 (1H, m), 7.37 (1H, d, J=
8.2 Hz), 7.30 (1H, dd, J=7.2, 8.0 Hz), 7.23 (1H, dd, J=7.5, 7.8 Hz), 3.75
(3H, s), 1.59 ppm (6H, s); 13C NMR (125 MHz, CDCl3) d=143.8, 137.1,
130.7, 130.1, 128.8, 128.5, 128.4, 128.3, 122.8, 120.7, 119.8, 109.7, 96.0,
76.5, 66.0, 31.7, 29.1 ppm; HRMS (ESI): m/z calcd for C20H19NNaO:
312.1357 ([M]+); found: 312.1359.
Synthesis of 3-(2-bromovinyl)-1-methyl-2-phenyl-indole (12): Wittig cou-
pling was carried out following a published procedure.[17] A solution of
potassium tert-butoxide (0.334 g, 2.98 mmol) in THF (5 mL) was added
to a solution of (bromomethyl)triphenylphosphonium bromide (1.30 g,
2.98 mmol) in THF (10 mL) at ꢀ788C. The resulting yellow solution was
added to formyl indole 11 (0.200 g, 0.85 mmol) in THF (20 mL) and stir-
red for 2 h at 208C. The reaction was quenched with H2O (5 mL), ex-
tracted with CHCl3 (20 mL), and washed with H2O (2ꢂ20 mL). The
product was purified by column chromatography (40–608C petroleum
ether/CH2Cl2 5:1) to give a colorless oil (0.215 g, 81%), which was a mix-
ture of the cis- and trans-isomers in a 1:2 ratio. 1H NMR (400 MHz,
CDCl3): d=7.85 (indole-H, trans-isomer), 7.81 (indole-H, cis-isomer),
7.2–7.57 (m), 7.12 (CC-H, cis-isomer, J=7.5 Hz), 7.07 (CC-H, trans-
isomer, J=14.2 Hz), 6.73 (CC-H, trans-isomer, J=14.2 Hz), 6.39 ppm
(CC-H, cis-isomer, J=7.5 Hz).
Synthesis of 2-phenyl cyanine alcohol (13): nBuLi(1.6m in hexanes,
0.400 mL, 0.641 mmol) was added to a solution of bromovinylindole 12
(mixed isomers) (0.200 g, 0.641 mmol) in THF (20 mL) at ꢀ788C. After
10 min, methyl benzoate (0.400 mL, 3.20 mmol) was added over 5 min,
and the solution was stirred for 1 h at ꢀ788C. The reaction was quenched
with H2O (5 mL), the product was extracted with CH2Cl2 (20 mL) and
washed with H2O (3ꢂ20 mL). The mixture was purified by column chro-
matography (40–608C petroleum ether/CH2Cl2/NEt3 6:1:0.1) to give the
Synthesis of 3-ethynyl-1-methyl-2-phenyl-indole (5b): Removal of the
protecting group was achieved by adapting a published procedure.[26] Pro-
tected acetylene indole 4b (0.450 g, 1.55 mmol) was dissolved in toluene
(75 mL), and freshly ground sodium hydroxide (1.24 g, 31.1 mmol) was
added. The mixture was stirred at reflux for 18 h. The solvents were re-
moved, and the residue was redissolved in CH2Cl2 (40 mL), washed with
water (3ꢂ40 mL), and dried over magnesium sulfate. After purification
by silica chromatography (40–608C petroleum ether/NEt3 1:0.05) and
evaporation of solvents, the product was obtained as a pale yellow oil
(0.140 g, 50%). 1H NMR (400 MHz, CDCl3): d=7.74 (1H, d, J=7.8 Hz),
7.62 (2H, d, J=7.0 Hz), 7.52 (2H, dd, J=7.2, 8.0 Hz), 7.45 (1H, m), 7.37
(1H, d, J=8.2 Hz), 7.30 (1H, dd, J=7.5, 7.0 Hz), 7.23 (1H, dd, J=7.0,
7.7 Hz), 3.75 (3H, s), 3.15 ppm (1H, s).
1
product (8.9 mg, 5%) as a pale yellow solid. H NMR (500 MHz, CDCl3):
d=7.88 (2H, d, J=8.0 Hz), 7.54 (2H, dd, J=8.5, 1.4 Hz), 7.35–7.44
(13H, m), 7.28–7.32 (4H, m), 7.17–7.23 (3H, m), 6.74 (2H, d, J=
16.2 Hz), 6.61 (2H, d, J=16.2 Hz), 3.62 ppm (6H, s); 13C NMR
(125 MHz, CDCl3): d=146.0, 140.3, 137.7, 132.0, 131.2, 130.9, 128.4,
128.3, 128.0, 126.8, 126.4, 125.7, 122.5, 122.2, 120.5, 120.4, 111.0, 109.6,
78.4, 46.1, 31.0, 29.7 ppm; HRMS (ESI): m/z calcd for C41H34N2O 570.72
([M]+; calcd for -OH: 553.26); found: 553.26.
Synthesis of 2-phenyl acetylene–cyanine alcohol (6b): Deprotected
acetylACHTUNGTRENNUNGene indole 5b (0.140 g, 0.606 mmol) was dissolved in THF (45 mL),
Synthesis of 2-phenyl cyanine trifluoroacetate (14+ CF3CO2ꢀ): The cya-
nine was formed in situ for NMR and absorption experiments by addi-
tion of trifluoroacetic acid (2% by volume with respect to solvent) to a
solution of the alcohol precursor 13 in CHCl3. 1H NMR (500 MHz,
CDCl3/2% [D]TFA): d=8.19 (2H, bs), 7.65 (2H, d, J=14.5 Hz), 7.53–
and the solution was freeze/thaw degassed. Methyl benzoate (0.038 mL,
0.30 mmol) and lithium bis(trimethylsilyl)amide (1.0m in THF, 2.42 mL)
were added, and the solution was stirred at 208C for 1 h. The reaction
was quenched with saturated aqueous solution of NH4Cl (10 mL), ex-
tracted with CH2Cl2 (25 mL), and washed with H2O (2ꢂ20 mL). Purifica-
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
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Cyanine-Like Dyes with Large Bond-Length Alternation
FULL PAPER
7.60 (8H, m), 7,48 (4H, dd, J=7.5, 7.2 Hz), 7.37–7.42 (3H, m), 7.32 (3H,
m), 7.28 (1H, s), 7.18 (2H, bs), 3.80 ppm (6H, s); 13C NMR (125 MHz,
CDCl3/2% [D]TFA): d=156.5, 139.7, 131.0, 130.4, 129.7, 128.9, 128.3,
126.9, 126.5, 125.5, 124.9, 122.1, 118.7, 112.0, 46.8, 32.6, 29.7 ppm; UV/Vis
(CHCl3/2% TFA): lmax (loge) 436 (3.59), 704 nm (4.84).
[14] K. J. Thorley, J. M. Hales, H. L. Anderson, J. W. Perry, Angew.
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Acknowledgements
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We thank the DARPA-MORPH (N00014–06–1–0897) and DARPA ZOE
(W31P4Q-09–1–0012) programs for financial support, and EPSRC Mass
Spectrometry Centre (Swansea) for mass spectra. The computational
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Computational Molecular Science and Technology”, funded through an
NSF CRIF award (Grant No. CHE-0946869) and by the Georgia Institute
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Received: February 16, 2013
Revised: May 7, 2013
Published online: && &&, 0000
Chem. Eur. J. 2013, 00, 0 – 0
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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These are not the final page numbers!